I had an old fridge in my basement with an ice maker and water dispenser. I want to hook it up to my bathroom sink. I have a bunch of spare 1/4" fittings in my toolbox. The only problem is my compression fitting that came with the fridge is this silver metal. My compression fittings are all brass. Previously that silverish compression fitting was connected to a copper line with a brass compression nut around it, and it worked just fine. I just want to make sure that I don't have a risk of a flood from bimetallic corrosion in the distant future.
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I believe you will find it is nickel plated brass, This is a common type of fitting and there would not be a problem. One of the largest problems for galvanic corrosion that I have seen is between copper and galvanized. A dielectric union helps but with a high mineral level in the water some corrosion can bee seen in just a few years. Most of the compression fittings for bathrooms & kitchen sinks are nickel plated brass it keeps them from growing green stuff on the brass and it is quite common.
Both should be brass & be perfectly fine for a proper & normal water plumbing system that doesn't have any regular steel, iron or aluminum in its components or connecting components. Those materials have been specifically relegated to natural gas & propane plumbing due to their inabilities to play nice.